The present invention relates to a method and apparatus for moistening webs of paper and paperboard during the different steps of paper manufacture.
In the manufacture of paper or paperboard, the fibrous material of the web is worked in plural ways under the action of the manufacturing equipment. The most important force effects are: tension imposed on the web, the pressure and heat applied for water removal from the web, and the pressure, heat and moistening used in calendering. The different coating and sizing techniques also cause certain effects, particularly in conjunction with the use a doctor blade or bar. The working of individual fibers and the web formed therefrom in the manufacturing process determines the properties and quality of the end product. The working of fibers is especially strongly affected by the moisture content of the fibers. Another factor contributing to fiber deformation is the temperature of the fibers. In combination, these variables determine the threshold point at which the deformation of the fibers remains permanent. Thus, the web temperature and moisture content can be modified so that wide variations can be effected in the qualities of the end product. On the other hand, changes in the web moisture content or temperature cause changes in web properties, e.g., variations in the cross-machine moisture content profile of the web lead to web thickness variations. This relationship may even be put into active service by controlling the web moisture content profile or temperature profile so that variations in the web thickness profile are equalized.
Concurrently, moistening a paper web is generally carried out with the help of steam boxes and water mist sprays. In the steam box, steam is directed to the web surface from a steam blow cavity forming a closed space with the exception of a slit opening toward the web. The steam condensing on the surface of the dry paper web moistens and heats the paper web. However, the efficiency of steaming degrades abruptly as the surface temperature of the web rises above 60 to 80° C., whereby condensation becomes lesser. In multinip calendering, for instance, the web temperature rises heavily due to the effect of the heated rolls and intensive fiber working forces.
While water mist spraying permits application of large amounts of water to the web, the typically applied amount of water is only 1-4 g/m2. As large water droplets spoil the web surface by causing defects such as mottling, water must be imported to the web surface in the form of finely atomized droplets. Conventionally, water is ejected from air-assisted nozzles capable of atomizing water into sufficiently small droplets. The water pumped to the nozzles is cold, and in practice the use of heated water is impossible, since the atomized water mist would cool down immediately after leaving the nozzle as atomized droplets. Nevertheless, heating the water to be atomized would theoretically be advantageous, particularly if large amounts of water need be applied. The use of hot water is, however, constrained by the rapid cooling is of the atomized water and evaporation thereof into the surrounding air of lower relative humidity. Due to these reasons, ejection of hot moistening water from conventional application nozzles is impossible.